G.R.A.S. 42AP Intelligent Pistonphone, Class 0

Typical applications and use

Reference calibration source

Precision microphone calibrations

Microphone comparisons

P-I index measurement at 250 Hz or 251.2 Hz

Calibration independent of atmospheric pressure and altitude

Design

It has a built-in precision barometer and a thermometer. Via its display and RS-232 interface, the user can read the actual corrected sound pressure level, as well as the pistonphone’s temperature and ambient static pressure.

With a microphone placed in the coupler of the pistonphone, the calibration level and frequency is nominally 114 dB* re. 20 μPa at either 250 Hz or 251.2 Hz. The actual sound pressure level, corrected for static ambient pressure, is shown on the display of the Pistonphone.

The display can also show the A-weighted sound pressure level after correcting it for using an A-weighting filter.

There is no flow control/handshaking; therefore commands must be sent one by one, waiting for each response. The input buffer is 32 bytes; in case of overflow, a response “Buffer overflow” will be submitted. This will not happen under normal conditions.

Commands and Responses

Two types of command are used. These are divided up as follows:

Interrogational commandsReturn information about the Pistonphone, its setup parameters, and measured ambient conditions

Syntax

Commands are not case sensitive.

All commands are executed by first typing inthe command then pressing the <Enter> key

Precision

The G.R.A.S. 42AP is an extremely stable laboratory standard sound source which can also be used for field calibrations – it retains its high accuracy even under hostile environmental conditions. It complies with all the requirements of IEC Standard 60942(2003) LS.

Couplers

The 42AP is delivered for calibrating ½" microphones directly since these are most commonly used. A 1" coupler (RA0023) for calibrating 1" microphones is also included.

Design

The pistonphone works on the principle of two reciprocating pistons actuated by a precision-machined cam with a sinusoidal profile. The rotation speed of the cam is controlled to within 0.1% via a tachometer signal in a feed-back loop.

The 42AP has a dual-colour LED above the ON/OFF switch to indicate both battery condition and stable operation.

When the pistonphone is operating properly, the LED shows green, indicating that the speed of the cam is correctly locked to give 250 Hz or, optionally, 251.2 Hz. If it shows red while the pistonphone is switched on, the speed is incorrect; most likely because of low batteries.

Operation

The operating procedure is straight forward, simply fit the microphone into the coupler of the pistonphone and switch on. The pistonphone will now produce a constant sound pressure level on the diaphragm of the microphone.

Compatibility

The Pistonphone 42AP is compatible with G.R.A.S. 1/2", 1/4", and 1/8" microphones and all other microphones having the same standard diameters.

Adapters are included for calibrating 1/4" and 1/8" microphones. Use the 1" coupler RA0023 for calibrating 1" microphones.

Precision

Each pistonphone is factory calibrated with an accuracy of ±0.09dB re. 20μPa and is supplied with an individual calibration certificate stating the exact value and test condition. The exact value is adjusted to be114 dB within ±0.05dB under reference conditions.

Since the output level of a pistonphone depends on the static ambient pressure, the 42AP has a built-in barometer which shows directly on a LCD the actual corrected sound pressure level. Adapters for the G.R.A.S. Environmental Microphone 41AL and Outdoor Microphone Systems 41AM and 41CN are available for use with a Pistonphone 42AP fitted with the 1" microphone coupler RA0023.